Even with actual emotional support animals, I don't understand why anyone needs a seventy pound dog as an emotional support animal. Is a twenty pound dog insufficiently supportive? Are there a certain number of support units per pound?

In everything there's a balance between personal freedom to do whatever you want, and the accommodation of other humans who may have conflicting needs or preferences. When there are such conflicting needs and preferences, I feel it's incumbent on the party imposing unusual requirements on others to minimize their imposition. When it comes to selecting an animal to accompany you on flights, this entails choosing an animal that is likely to fit comfortably, not be unusually odorous or violent, and otherwise be relatively inoffensive to one's fellow passengers, at least to the extent one is able. (I understand that only certain breeds of dog are suitable as seeing eye dogs, and for that I will gladly make an exception, as these animals are usually exceptionally well-trained and are also almost necessary for their owners to function).